In communication devices within a communication network with a typical layer-2 redundant configuration, STP (Spanning Tree Protocol) is used and, in order to prevent loops of packet forwarding, blocking ports are formed. However, in communication devices that use STP, although loops can be prevented, paths that connect to blocking ports are unusable; therefore, it is difficult to effectively use the used band.
Therefore, in recent years, TRILL (Transparent Interconnection of Lots of Links), which is a layer-2 redundancy protocol, has been standardized according to RFC6325. TRILL is the technology for redundancy of paths in a communication network, such as Ethernet (registered trademark). Furthermore, TRILL has a mechanism for preventing loops without forming blocking ports; thus, compared to STP, an effective use of the used band and high-speed path switching are possible.
TRILL is the technology that achieves redundancy and high-speed switching of paths in a communication network by applying, to a layer 2, IS-IS that is a layer-3 routing protocol. TRILL-compatible communication devices with a built-in L2SW function have a function to determine a path with the shortest distance by using the SPF (Shortest Path First) method among physical paths in a TRILL network that is made up of multiple communication devices.
Furthermore, as each communication device in a TRILL network recognizes costs of each physical path in the TRILL network, it has a function to autonomously select the least-cost path from multiple assignable redundant paths when the currently used physical path in the TRILL network becomes unusable due to a failure, or the like.
Furthermore, TRILL has the ECMP (Equal Cost Multi Path) function as a system for assigning redundant paths in a distributed manner in the unit of traffic flows when there are multiple redundant paths with equal costs between the terminal communication devices in a TRILL network. Moreover, as RFC does not define a distribution method of the ECMP function, it is possible to apply a method of assigning traffic flows to redundant paths in the unit of VLAN (Virtual Local Area Network) in terms of the layer 2.
When each communication device assigns physical paths on a per VLAN basis, it needs to perform assignment operations, such as software processing, e.g., recalculation of physical paths on a per VLAN basis, or hardware processing for an access to hardware on a per VLAN basis, and the operations are time-consuming. Furthermore, the processing time of software processing is about 2 ms; however, hardware processing corresponds to various operations, such as port settings or frame forwarding rule settings, and its processing time is about 20 ms. That is, hardware processing needs a large amount of processing time compared to software processing.    [Patent Literature 1] Japanese Laid-open Patent Publication No. 2007-325271
However, in communication devices within a TRILL network, when the number of VLANs is increased, the number of times an assignment operation is performed is increased in proportion to the number of VLANs, and its processing time becomes longer. As TRILL enables the settings of VLANs of, for example, up to 4096, each communication device needs an assignment operation 4096 times at a maximum, and the effect on the processing time is large. Furthermore, as TRILL FGL (Fine Grained Labeling) enables the settings of VLANs of, for example, up to 16 millions, it needs an assignment operation 16 million times at a maximum, and the effect on the processing time needed for the assignment operation is large. Therefore, in communication devices, for example, after a path failure is detected, a setting operation is performed on a per VLAN basis; therefore, a large amount of time is needed for path switching and re-establishment and, meanwhile, there is a possibility that communication services are interrupted.